1. Field of the Invention
The present invention relates to a combustion pressure sensor for detecting a combustion pressure in the combustion chamber of an internal combustion engine such as a diesel engine.
2. Description of the Related Art
Combustion pressure sensors adapted to be mounted to an internal combustion engine such as a diesel engine for detecting the combustion pressure in a combustion chamber of the internal combustion engine are known and used heretofore. In one example disclosed in Japanese Patent Application Publication JP 2005-90954 A (corresponding to US 2005/0061063 A1), the combustion pressure sensor is formed as an integral part of a glow plug. As shown in FIG. 8 hereof, the disclosed combustion pressure sensor 9 includes a pressure transmitting member 91 (serving also as a heating rod) slidably displaceable in the axial direction relative to a housing 92 to transmit a pressure in the combustion chamber to a pressure sensor 93 disposed on a rear end of the combustion pressure sensor 9 for the detection of a combustion pressure in the combustion chamber.
In order to prevent entry of hot combustion gas from between the pressure transmitting member (heating rod) 91 and the housing 92, the combustion pressure sensor 9 further has a seal member 94 and a membrane 95. To this end, the seal member 94 and the membrane 95 are arranged to tightly block or seal a clearance between the pressure transmitting member (heating rod) 91 and the housing 92. The membrane 95 is connected by welding to the housing 92 and the pressure transmitting member 91 so that the hot combustion gas can be trapped within a front end portion of the housing 92.
With this arrangement, however, due to contraction in shape of the housing 99 that may occur in the axial direction of the housing 92 when the combustion pressure sensor 9 is mounted to the internal combustion engine, the pressure transmitting member 91, which is connected to the housing 92 via the membrane 95, is also displaced in the axial direction relative to the housing 92. As a consequence of this displacement, a force or pressure is transmitted to the pressure sensor 93, which will cause an unwanted change in the sensor output. This may lead to fluctuation in the initial value of the combustion pressure sensor 9 and deterioration of the sensor accuracy. To secure application to the engine control purposes, the combustion pressure sensor 9 requires collection using another sensor, which will incur additional cost. In some cases, engine control is rendered difficult to achieve.
Another conventional combustion pressure sensor is disclosed in Japanese Patent Application Publication JP 2006-084468 A2 (corresponding to US 2006/0053875 A1), which includes a seal in the form of a bellows-shaped component provided between the pressure transmitting member (heating rod) and the housing. However, since the seal is disposed in a front end portion of the housing, the seal is likely to cause thermal deterioration under the effect of high-temperature combustion gas. Accordingly, in a severe environment, sealing property between the pressure transmitting member and the housing tends to deteriorate, making it difficult to provide a sufficient level of durability.